Well, as I see it, given some historical variation, striding (advancing rapidly in some sense) and striving (actively working towards a goal) are quite similar in meaning. As someone said in this comment thread, strijd (fight) and stride have the same ancestor so the meanings are close enough to be related.

I do not think so, it is likely that this piece of propaganda was from before the Netherlands was invaded by Germany, since it is advertising the Dutch own Nazi movement. When the Germans came they did not approve of the idea of an independent Fascist Netherlands so I don't think they could've published these posters after the invasion. Japan attacked the colonies quite a bit after the homeland was already lost so the dates do not add up.